The present invention relates to a method of processing the surface of a specimen and an apparatus capable of performing this method; and, in particular, the invention relates to a method and apparatus suitable for plasma-etching the surface of a specimen on which semiconductor elements and the like are to be formed.
An apparatus for etching semiconductor elements, for example, an apparatus employing a co-called ECR (Electron Cyclotron Resonance) system, will be explained. This ECR system generates a plasma by exciting an inert gas in a vacuum container to which microwave energy and a magnetic field have been applied from the outside. The magnetic field causes electrons to move with a cyclotronic motion. The cyclotron frequency and the microwave frequency in resonance produce an environment in which a plasma can be generated efficiently. To accelerate plasma particles (ions) and cause them travel fast enough before striking a target in their path, a high-frequency voltage is applied to the target. A halogen gas, such as chlorine gas or fluorine gas, is used for generation of a plasma gas.
A high-precision type surface treating apparatus is disclosed in Japanese Non-examined Patent Publication No. 06-151360 (1994). This patent publication discloses that the intermittent on/off control of a high-frequency voltage applied to the target increases the selectivity of a surface substance (silicone) to be etched from the ground (oxide film) of a target and makes the etching rate less independent of conductor patterns. Further, in Japanese Non-examined Patent Publication No. 62-154734 (1987), there is disclosed a method of intermittently turning on and off a high-frequency voltage and etching slanted areas with a high-depositing etching gas. Furthermore, in Japanese Non-examined Patent Publication No. 60-50923 (1985), there is disclosed a method of intermittently turning on and off a high-frequency voltage according to the supply quantity of an etching gas to increase the anisotropy. Furthermore, U.S. Pat. No. 4,585,516 discloses a 3-electrode etching apparatus and a method of intermittently turning on and off a high-frequency voltage across two of such electrodes to assure a uniform etching speed over the whole wafer.
Along with a recent trend toward finer patterning of semiconductor elements, a problem of damage of semiconductor devices caused by the plasma used in the processing thereof is becoming significant and has been drawing considerable attention. More specifically, a typical thickness of a gate oxide film of a metal oxide semiconductor (MOS) has become less than 6 nm in memory devices with the introduction of the 256 M device. In addition to the demand for a thinner gate oxide film, when the aspect ratio (a ratio of vertical to lateral directions) in the processing becomes greater, the electrical damage caused by a so-called electron shading phenomenon becomes substantial. With reference to FIGS. 24(1) and 24(2) of the accompanying drawings, this electron-shading phenomenon will be more particularly described. FIG. 24(1) is a cross-sectional view of a semiconductor wafer exposed to plasma within an etching apparatus. FIG. 24(2) is a plan view of a resist pattern on the wafer shown in FIG. 24(1) as seen from above. A device insulation oxide film 204 and a gate oxide film 203 are formed on a Si substrate 205, and then, on these films, a poly-Si layer 202 and a resist 201 are formed in a comb pattern. During plasma etching, electrons 206 and ions 207 are bombarded on the specimen. Ions 207, which are accelerated by a high frequency voltage applied to the specimen, impinge on the surface of the specimen directly in the vertical direction. Electrons 206, which have a small mass and therefore have random speed components impinge on the specimen in random directions. Therefore, for processing of the surface with a groove having a high aspect ratio, as shown in FIG. 24(1), although ions can reach the bottom of the groove 208, most of the electrons are captured on the side walls of the resist 201. Then, positive charges are accumulated in gate oxide film 203 via poly-Si layer 202, and when the amount of this charge exceeds a predetermined value, the gate oxide film 203 is caused to breakdown, thereby resulting in a device failure. This phenomenon that prevents electrons from being supplied to the bottom of a fine patterned groove due to a difference in the directivities of ions and electrons is called electron shading.
Further, as smaller semiconductor elements have been required, finer patterning and working of them has become essential. For example, recent semiconductor circuit patterns have lines and spaces (which are equivalent to wires and electrodes on semiconductor elements) of 0.3 microns or narrower. However, the conventional etching processes cannot satisfy such a fine patterning requirement. Necessarily, in the resulting product, the etched lines are wider than required and resulting patterns are undesirable. Further, the etching status is greatly affected by a difference between the speed of fine-line etching and the speed of wide-space etching and a difference in the shapes (shape micro loading) Furthermore, since the oxide film of a gate of a MOS (Metal Oxide Semiconductor) transistor (for memory chips of 256 MB or higher) is very thin (6 nm or less), its etching status is greatly affected by the anisotropy and the ratio of ground selectivity (ratio of selecting an oxide film as the ground) which are inversely-proportional to each other.
Therefore, an object of the present invention is to provide for a surface processing method and an apparatus, which can substantially reduce the damage to a semiconductor device caused by this electron shading.
Another object of the present invention is to provide a surface treating method and apparatus which can increase the anisotropy and the ratio of ground selectivity in fine pattern etching processes.
The one object of the invention can be accomplished by provision of a fine pattern etching processing method which is performed by applying a high frequency voltage to the specimen, and which is comprised of repeating the steps of: turning off the high frequency voltage applied to the specimen before a charged voltage of the pattern reaches an insulation breakdown voltage of the gate oxide film to which the pattern is connected; and turning on the high frequency voltage when the charged voltage of the pattern becomes sufficiently low.
The other object of the invention can be accomplished by carrying out a process of fine pattern etching using a surface treating apparatus comprising a vacuum chamber, a means for generating a plasma in said chamber, and a high-frequency power supply which applies a high-frequency voltage across a target wafer and a target table which holds a target wafer to be etched by the plasma; wherein the amplitude of the high-frequency voltage is increased to improve the wall-to-bottom perpendicularity in etching and the high-frequency power supply is controlled so as to be turned on and off intermittently.